This invention relates to a process for treatment of viscous products, in particular for performance of polymerization processes, in particular for homo- or copolymerization of thermoplastics and elastomers, wherein monomer(s) and/or catalysts and/or initiators are added to a backmixed mixing kneader, in particular having a length/diameter ratio of 0.5-3.5, heat is supplied to the product, and backmixed with already reacted product, and the reacted product is withdrawn from the mixing kneader, and to apparatus therefor.
A considerable proportion of polymerization reactions, in particular for production of homo- and copolymeric thermoplastics and elastomers, are commercially performed as a slurry or solution process in one or more series-connected, continuous-flow, backmixed, vertical stirred tank reactors known as CSTRs.
These stirred tank reactors have the task of ensuring that the monomers, the catalysts and initiators be distributed in a solvent/diluent under precisely defined processing conditions, such as temperature and pressure, as homogeneously as possible in order that the reaction may proceed in a policed manner, that a uniform product quality having the desired molar mass may be formed and that, in addition, the heat of reaction may be controlled.
The problem with these stirred tank reactors, then, is that only products having a low apparent viscosity can be processed. As the concentration of the polymer in the solvent/diluent increases, the apparent viscosity of the reaction mass increases to such an extent that the stirrer ultimately cannot generate sufficient convective flow. The consequence thereof is an inhomogeneous distribution on the part of the monomers. This leads to clumping, poor molar mass distribution, caking, local overheating up to and including a runaway reaction for the entire reactor contents.
A further problem with stirred tank reactors is that some products give rise to foaming, which can lead to blockages in the vapor outlet ports.
The abovementioned processing risks explain why stirred tank reactors can only be operated with a large excess of solvent/diluent of up to about 90% of the reaction mass, or only conversions of less than 50% are achievable in the case of bulk polymerizations. As a consequence thereof, additional operations become necessary for mechanical/thermal removal of the diluent/solvent/monomer or for postreaction. This is generally accomplished in dewatering screws, evaporation and drying systems, and also ripening tanks. They require high capital, energy and operating costs. Moreover, there are new polymers which are not processable using a water stripping process.
Bulk polymerizations are also performed continuously in single- or multi-shaft extruders (for example from Werner Pfleiderer, Buss-Kneter, Welding Engineers, etc.). These apparatuses are designed for polymerizations in the viscous phase up to high conversions. They are constructed as continuous plug-flow reactors and accordingly have a large L/D ratio of from >5 to about 40.
Here the following problems arise:                a) In the case of slow polymer reactions with reaction times >5 minutes during which the reaction mass remains in the liquid state for a long period, plug flow cannot be maintained. The very different rheological properties between the monomers and polymers prevent uniform product transportation, and this leads to undesirable fluctuations in quality.        b) The substantial exothermicity of many polymerization processes and also the dissipated kneading energy frequently make it necessary to remove these energies via evaporative cooling. In evaporative cooling, some of the monomer or of an admixed solvent/diluent is evaporated, condensed in an external condenser and returned as condensate into the reactor. Owing to the large L/D ratio and the large screw cross-section necessitated by the design, only very limited free cross-sectional areas are available for the withdrawal of vapors. This leads to the undesirable entrainment of polymers into the vapor lines and into the reflux condenser and, as a consequence thereof, to blockages.        c) An additional complicating factor with the production of (co)polymers from two or more different monomers is that it is mainly the monomer which has the lowest evaporating point that evaporates for the evaporative cooling, so that there is a shift in the monomer concentrations in the reactor, in particular in the region of the entry orifice for the condensate reflux. This is generally undesirable.        d) It is also disadvantageous that the free product volume of screws is limited to about 1.5 m3 for mechanical engineering reasons, so that only low throughputs can be achieved in the case of reactions having residence times >5 minutes, which requires the installation of a plurality of parallel lines at correspondingly high capital and operating costs.        
A further way of performing bulk polymerizations up to high conversions is described in U.S. Pat. No. 5,372,418. Here, co- or contrarotating multi-screw extruders having nonmeshing screws, or pairs of screws, which convey in opposite directions are described for the polymerization of monomers by backmixing with the polymer in the viscous phase. These apparatuses are in principle capable of performing polymerization processes up to high conversions and at the same time of avoiding the above-described disadvantages a) (collapse of plug flow) and c) (recipe shift through reflux) of the plug-flow extruder. However, the above-described problems b) (reduced free cross-section) and d) (capacity) still remain unsolved.
The abovementioned processes are also carried out in so-called mixing kneaders in which appropriate kneading and transporting elements transport the product from an inlet to an outlet and at the same time ensure that the product comes into intensive contact with the heat-exchanging surfaces. Such mixing kneaders are described for example in DE patent 23 49 106, EP 0 517 068 A1 and DE 195 36 944 A1.